1. Field of the Invention
Methods and devices consistent with the present invention relate to a structure and method of controlling precession when drilling and, more particularly, to controlling precession through the unbalanced radial biasing of blades and the use of free sliding axial blade contacts in a fixed stabilizer.
2. Description of the Related Art
In a drilling assembly for drilling for oil and the like, a “non-rotating” part may be used which does not rotate with the drill bit. For example, a non-rotating stabilizer may be used. However, although the non-rotating stabilizer does not rotate along with the drill bit, the non-rotating stabilizer may rotate due to precession because of other forces associated with drilling, such as lateral and axial forces. In at least some instances it may be advantageous to control the non-rotating stabilizer, or some other non-rotating part, so that it does not rotate due to precession. One environment in which it can be beneficial to limit the rotation of a non-rotating stabilizer is when the non-rotating stabilizer is used in a directional drilling assembly.
In related art, there are proposed methods for controlling the direction of drilling, such that the drill bit may be moved from vertical drilling to drilling in a particular direction. One method for accomplishing directional drilling is shown by U.S. Pat. No. 5,931,239 (“the '239 patent”), which is incorporated herein by reference. In the '239 patent, the direction of drilling is controlled by extending and retracting stabilizer blades in an adjustable stabilizer portion of a non-rotating stabilizer. In a non-limiting embodiment of the patent, there are four such stabilizer blades. When one of the stabilizer blades is extended and the opposite blade is retracted, the drilling assembly drills towards the retracted stabilizer blade (and away from the opposing extended stabilizer blade).
However, rotation of the non-rotating stabilizer can cause problems with the directional control. Particularly, because the adjustable blades which control the drilling direction rotate along with the non-rotating stabilizer, when the non-rotating stabilizer rotates, the shifted position of the adjustable blades changes the direction in which the blades urge the drilling assembly. For example, to turn the drilling bit of the drilling assembly in a left direction, a left blade is retracted and a right blade is extended. If the non-rotating stabilizer then rotates a half-turn (180 degrees), the position of the blades are switched. Accordingly, the originally retracted blade moves from the left side to the right side and the originally extended blade moves from the right side to the left side. In this manner, rotation of the non-rotating stabilizer moves the blades to a position of turning the drilling assembly to the right rather than the left. It is thus difficult to control drilling to proceed in a particular direction when the non-rotating stabilizer rotates due to precession.
The drilling apparatus can be programmed to adjust the blades as the non-rotating stabilizer turns in order to counteract the rotation. However, if the non-rotating stabilizer turns too quickly, adjustments to the blades cannot keep pace with the rotation. Furthermore, controlling the direction of the drilling is easier if the non-rotating stabilizer turns slower or not at all.
Accordingly, it would be advantageous to be able to limit the precession of a non-rotating part such as a stabilizer.